Not Applicable.
Not Applicable.
1. Field of the Invention
The invention relates to a method and an arrangement for improving leak tolerance in an earpiece. The invention can be applied preferably in teleterminals, particularly in mobile stations.
2. Description of the Related Art
Teleterminals conventionally contain a receiver part which has an earpiece for reproducing the received acoustic signal. The earpiece has been conventionally designed in such a way that it forms the maximum sound volume and the best quality of sound when the earpiece is sealed against the user""s ear. If there is a gap, i.e. a leak between the earpiece and the user""s ear, this usually causes a significant weakening of the sensed sound pressure. Additionally, the frequency distribution of the sensed sound does not then correspond to the original acoustic signal but low frequencies are attenuated to a greater extent than high frequencies. The ability of an earpiece to maintain its acoustic properties when the gap between the earpiece and the ear changes, is called its leak tolerance.
The problem described above is extremely serious particularly in mobile stations, because the mobile station is rarely completely sealed against the user""s ear. On the other hand, standards relating to mobile stations are primarily based on measurements where the gap between the mobile station and the artificial ear has been arranged so that there is a tight seal. In order to ensure that the volume and frequency distribution of the reproduced sound are according to the specifications also in real operating conditions, extremely good leak tolerance is required from the earpiece arrangement.
To improve leak tolerance the following ways are prior known. The leak tolerance can be improved by arranging a loose coupling to the membrane which produces the sound waves in the earpiece capsule and by loading it by a relatively large volume situated behind the earpiece capsule. Most preferably the volume behind the earpiece capsule has been arranged to be open, in which case the aforementioned volume becomes as large as possible. Another way to improve the leak tolerance is to lower the acoustic output impedance of the arrangement by using an acoustic return path.
FIG. 1 shows an earpiece 100 according to prior art. It comprises an earpiece capsule 101 which converts an electric signal into an acoustic sound. The earpiece capsule 101 is connected to the housing 103 of the earpiece by its edges 102. The sound wave formed by the earpiece capsule is generated in the volume 104 between the earpiece 101, the edges 102 and the housing 103, from which it is transferred to the external volume of the housing through holes 105. Between the housing 103 and the ear 106 there remains a volume 107 which thus in an optimal situation is closed. If the earpiece is a part of a mobile station, the housing 103 is preferably the cover of the mobile station.
In the solution shown in FIG. 1, leak tolerance has been improved by arranging an acoustic return path from the back part of the earpiece capsule to the front part of it. The acoustic return path is formed of the holes 114 in the back part of the earpiece capsule, the volume 113 behind the earpiece capsule, the volume 112 on the sides of the earpiece capsule and the holes 115 in the front edge of the earpiece capsule. The volume arranged for said acoustic return path has been closed by a special casing 110 in the solution of FIG. 1, but the volume can also be formed by a normal casing of a device, such as a mobile station, and the components inside it. In the solution shown in FIG. 1 an acoustic volume 113 has been arranged for the acoustic path and it can contain material attenuating high frequency components. The ability to improve leak tolerance in the solution shown in FIG. 1 is based on the fact that the return path arranged especially for low frequencies operates as an acoustic load for the earpiece capsule at low frequencies, in which case the changes in external load have a smaller relative effect on the acoustic total load of the earpiece capsule.
A disadvantage of the above described solution according to prior art is that the load caused by the acoustic return path is difficult to optimize. The leak tolerance to be achieved depends essentially on the size of the volume arranged behind the earpiece capsule. In small-sized devices, such as mobile stations, it is often impossible to provide a sufficiently large volume to achieve the optimum acoustic load.
FIG. 2 shows an acoustic equivalent circuit of an earpiece according to FIG. 1. In it, the earpiece capsule forms a pressure wave by operating as an acoustic source 201 and comprising an internal impedance 204. The pressure wave propagates to the outside of the earpiece arrangement through holes in the casing, such that the holes form an impedance 205 and the external volume forms a load impedance 206. The interface between the earpiece and the external volume has been marked by 207 in FIG. 1. The acoustic return path operates as a feedback impedance 215. The load impedance 206 consists mainly of the load caused by the ear and the load resulting from the leak between the earpiece and the ear. It can be noted from the equivalent circuit that the changes in the load impedance 206 have a major impact on the acoustic power which is transferred to the load, and by means of the feedback impedance, the effect of load variation can be reduced to only a minor extent.
The aim of the present invention is to devise an earpiece solution which achieves good leak tolerance in a small-sized device, such as a mobile station.
One idea of the invention is that the acoustic return path is directed from the back part of the earpiece capsule to the volume between the earpiece and the user""s ear. By means of the solution according to the invention, an optimum, controlled load especially for low frequencies is achieved, in which case a change in the volume between the earpiece and the ear only has a minor effect on the volume and the frequency distribution of the sensed sound. By means of the solution a good leak tolerance is achieved, though the volume to be arranged behind the earpiece capsule is small.
A method according to the invention for improving leak tolerance in an earpiece such that the sound formed by an earpiece capsule is directed through the first acoustic path to the first volume which is confined by the user""s ear and the housing part between the earpiece capsule and the ear, is characterized in that from the back part of the earpiece capsule a sound formed by the earpiece capsule is directed to said first volume through the second acoustic path.
An arrangement according to the invention for improving leak tolerance of an earpiece, which comprises
an earpiece capsule,
a housing part situated between the earpiece capsule and the user""s ear for confining the first acoustic volume between said housing part and user""s ear and
the first acoustic path arranged between the front part of the earpiece capsule and the first acoustic volume for directing sound from the front part of the earpiece capsule to the first acoustic volume,
is characterized in that the arrangement comprises additionally
the second acoustic path arranged from the back part of the earpiece capsule to said first volume.
A mobile station according to the invention, whose earpiece comprises
an earpiece capsule,
a housing part situated between the earpiece capsule and the user""s ear for confining the first acoustic volume between said housing part and user""s ear and
the first acoustic path arranged between the front part of the earpiece capsule and the first acoustic volume for directing sound from the front part of the earpiece capsule to the first acoustic volume,
is characterized in that the earpiece comprises additionally
the second acoustic path arranged from the back part of the earpiece capsule to said first volume for improving leak tolerance.
Preferable embodiments of the invention have been presented in dependent claims.
By the front and back parts of the earpiece capsule one means herein the front and back parts of a membrane which forms sound waves and is situated in the earpiece capsule, and sound waves generated in these front and back parts are in opposite phases from each other.
By the earpiece one means herein the earpiece capsule and acoustic and mechanical structures connected to it.